Selection Pressure in Breeding
The principal objective of selective breeding is to maximize the pairing of good genes by breeding dogs not affected with (and preferably, not susceptible to) CHD.
For the most rapid genetic change, the breeder can decide to mate only the tightest-hipped dogs within the breed (those with the lowest DI) and then continue to inbreed for tight hips from there. This approach, however, will create increased inbreeding. Founding a breeding program on only a few dogs, and inbreeding on these dogs, would reduce the overall genetic diversity in the gene pool and could contribute to the loss of some desirable traits or lead to the expression of some undesirable traits. This reality affects some breeds more than others. For example less than 5% of Golden Retrievers have hip laxity in the ‘tight-hipped’ range, meaning a DI below 0.30. If one were to require that breeding candidates conform to this standard, 95% of the Golden Retrievers would be excluded from breeding, resulting in a serious reduction in genetic diversity. This breeding strategy would neither be practical nor acceptable to breeders and is not recommended by PennHIP.
To avoid these potential problems accompanying ‘extreme’ selection, PennHIP suggests a more ‘moderate’ approach which goes hand in hand with the PennHIP testing. Particularly in breeds with few or no members having tight (OA-unsusceptible) hips this moderate approach is preferable. In such breeds it is recommended that breeders choose breeding stock from the tightest 40% of the breed (meaning the 60th percentile or better), thereby maintaining an acceptable level of genetic diversity while still applying meaningful selection pressure. By breeding only dogs with hips above the breed average (60th percentile or better) the overall breed average will move toward better (tighter) hips from one generation to the next. Clearly the more selection pressure applied (stricter selection criteria), the more rapid the genetic change.
The PennHIP database ranks each dog relative to other members of the breed making it possible for the breeder to identify dogs whose DI will apply meaningful selection pressure. By applying at least moderate selection pressure, eventually the average of the population will shift with each generation toward tighter hips, increasingly tightening the minimum standard for breeding. By following these time-tested principles of quantitative genetics, ultimately fewer dogs will be at risk for developing OA. Understandably, more rapid genetic change could be achieved by imposing greater selection pressure or by using estimates of breeding value (EBV) from incorporation of the pedigree. These strategies are recommended for the aggressive breeder wishing to achieve the most rapid hip improvement.